Try mixing up different sets of tubes. I have bugle boys in the front, tesla's in the middle and JAN's in the back. I listened for hiss and microphonics with my headphones each time I changed out a set. This combo sounds much warmer and musical than the stock Sovtek set and is even quieter than the original set. BTW, there is no need to leave the preamp in standby all the time. This just uses the tubes up faster. Just turn the preamp on standby the day you will be listening or on the weekends, etc.
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There are a lot of threads on this if you search the archives. This is copy/paste from a post to the same question I gave my opinion on a year ago:
A local Line1 guru who spent a year trying every NOS tube out there saved me the effort, I went with his recommendations, though have tweaked the sound depending on which tubes are in which locations. Tungsrams give an almost-startling amount of image dimensionality and sizing, a smoother treble while still retaining the solid-state bass slam of the Line1. Electro-Harmonix are a slightly faster cleaner sounding Sovtek. I use the Tungsrams in the back (V1) and front (V3), with the EH in the center (V2) locations. Depending on your tastes, running the EH in both V2 and V3 may sound better, and be cheaper too (EH are in production at about $12, tungsrams can be $40+ ea). My understanding with Bugle Boys would be to use them in either the front or rear only in combination with 4 EH. I haven't yet heard this combo, or for that matter any of the 100 other combos that others can offer up. But I am quite happy with the sound of the setup I described...
Try Amperex PQ white label early 60s USA made 6922 or 7308 tubes. Also Siemens early 60s CCa.
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Who knows??? I am new to this also, having just purchased and tubed a SF Line 2 to try something different compared to my Muse 3 Signature SS preamp. Sort of a mixed bag. I talked with one "well regarded on the net" tube vendor who basically said it is almost impossible to adequately tube a SF Line preamp, and that he doesn't try anymore, along with some generally negative remarks about the 6922/6Dj8 tube. I was discouraged and called Chris Johnson at Parts Connexion, who designed the SF Line preamps, and who has successfully recommended parts I have purchased before. Chris suggested putting your best tubes in the two back spots, and I ended up with Siemens CCa in those spots and Siemens 6922 nickel pins in the other four spots. With only a short break-in, the MF and HF sound good, but the LF is a bit loose or boomy, compared to my Muse pre, which resulted in very tight well defined bass (my speakers go down into the mid-low 20's. In general, the tube pre provides better texture and three-dimensionality compared to the SS, but the SS pre has better resolution and much better defined bass. I am going to ride this out for awhile until the tubes break-in, then make up my mind whether to stay with the SS or tube preamp.